Generally speaking, a bearing comprises an inner ring and an outer ring adapted to rotate around a rotation axis, one with respect to the other. In a plain bearing, the two rings are in sliding contact. In a rolling bearing, several rolling bodies are installed between the two rings. These rolling bodies can be balls, rollers or needles. Thus, a rolling bearing can be, for instance, a ball bearing, a roller bearing or a needle bearing.
In the field of bearings, it is known to use a tachometer in order to determine the rotation speed of a member supported by a bearing. As explained in EP-A-1 933 155, one can use an encoder washer with magnetic poles, fast in rotation with a rotatable ring of a bearing, and several sensors evenly distributed around the encoder washer and mounted onto a foldable printed circuit board installed within an outer body. The accuracy of the detection obtained by the respective sensors depends, amongst others, on the thickness of a radial air gap defined between the encoder washer and each sensor. In case of excentration of the sensors, that is if they are located at different radial distances with respect to the rotation axis of the encoder washer, the respective air gaps defined between the encoder washer and the sensors have different thicknesses, which implies that they detect different values for a rotating magnetic field generated by the encoder washer. This might lead to perturbations in the detection of the rotation of the encoder washer. The same applies if the rotation axis has transverse movements during rotation, for instance in case of unbalance of the rotatable ring.